1. Field of the Invention
The present invention relates to a fixing apparatus to be incorporated into a copying machine, a laser beam printer or the like.
2. Description of the Related Art
A conventional fixing apparatus provided with a fixing roller and a pressing roller fixes an unfixed developer image onto a recording medium by holding a recording medium that has a multiple-color/multiple-layer or monochrome unfixed developer image on it and transferring the recording medium through their pressure-contact portion between its fixing roller and pressing roller. FIG. 21 is a schematic sectional view of a heat roller fixing apparatus that is used as a toner image fixing apparatus in a color image forming apparatus such as an electrophotographic apparatus, an example of such a conventional fixing apparatus.
In FIG. 21, a fixing roller is provided with a silicone rubber layer on an aluminum core shaft. The silicone rubber is of the RTV or LTV type and has an elasticity as represented by a JIS-A Standard rubber hardness within a range of from 20.degree. to 25.degree.. A larger thickness of the silicone rubber leads to a lower surface hardness of the roller, and hence to a higher fixability. When the thickness becomes excessively larger, in contrast, heat conduction requiring a longer period of time leads to a longer warm-up time, or makes it impossible to achieve uniform fixing.
On the other hand, the pressing roller suffices to have a smaller elasticity than that of the fixing roller: provision of an HTV or fluorine rubber layer on the aluminum core shaft will do, or a PFA or PTFE may be coated on the surface.
When sufficient heat and pressing force are applied to the roller with a view to obtaining an image of a high fixability by the use of this conventional fixing apparatus, the surface of a toner image becomes smoother and gloss of the image is improved.
However, when an image formed on a copy paper of a low gloss has a high gloss, there occurs a large difference in gloss between the non-image portion and the image portion. This gives a feeling of mismatch to a viewer. Particularly, in the case of a monochromatic character image in a business use, the character itself has a high gloss, making it difficult to read the character under the effect of reflection.
It is therefore proposed to reduce gloss by decreasing the fixing temperature or reducing the pressing force to obtain a low-gloss image. However, on the other hand, a lower fixing temperature leads to more difficult fixing of toner, and particularly in the case that full color or four color toners are applied, it may become impossible to achieve fixing.
In the conventional fixing apparatus, therefore, it is difficult to obtain an image having a low gloss and a high fixability.
With a conventional roller, if the fixing roller and the pressing roller are left in pressure contact with the heater turned off, i.e., not heated by the heater for a long period of time (for a night, for example), the roller collapsed at the pressure-contact portion as shown in FIG. 21 would harden, and this deflection would not be eliminated even by heating again with the heater. This causes abnormal sound during rotation of the rollers. A larger deflection leads to a problem of occurrence of a defective image caused by irregular rotation.